Semiconductor device with metal carrier and manufacturing method

ABSTRACT

Semiconductor device including a metal carrier substrate. Above the carrier substrate a first semiconductor layer of Al x1 Ga y1 In z1 N (x1+y1+z1=1, x1≧0, y1≧0, z1≧0) is formed. A second semiconductor layer of Al x2 Ga y2 In z2 N (x2+y2+z2=1, x2&gt;x1, y2≧0, z2≧0) is arranged on the first semiconductor layer and a gate region is arranged on the second semiconductor layer. The semiconductor device furthermore includes a source region and a drain region, wherein one of these regions is electrically coupled to the metal carrier substrate and includes a conductive region extending through the first semiconductor layer.

BACKGROUND OF THE INVENTION

Power semiconductor devices such as Field Effect Transistors (FETs) and High Electron Mobility Transistors (HEMTs) are widely used for applications such as power switch circuits. Examples for requirements on these devices are low area specific on-resistance R_(ON)×A, high breakdown voltage V_(BR), and high robustness under electrical breakdown conditions.

Power semiconductor devices based on wide band gap semiconductor materials such as GaN allow for low specific on-resistance. Reduction of the specific on-resistance is accompanied by requirements on improved heat dissipation and improved device robustness.

A need exists for a nitride semiconductor power device having improved heat dissipation and improved device robustness

For these and other reasons there is a need for the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and together with the description serve to explain principles of embodiments. Other embodiments and many of the intended advantages of embodiments will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts. The features of the various illustrated embodiments can be combined in any way unless they exclude each other. For differentiation of different layers, a numbering such as first layer, second layer and third layer is used. The numbering only serves to distinguish between these layers and is independent from any sequence of manufacture.

FIG. 1A is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a nitride semiconductor power device including a carrier substrate made of metal.

FIG. 1B is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a nitride semiconductor power device including a carrier substrate made of metal and source regions electrically coupled to a contact pad arranged on top of an active area of the device.

FIG. 2 is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a nitride semiconductor power device including a carrier substrate made of metal and a doped nitride semiconductor layer for fixing avalanche breakdown.

FIG. 3 is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a nitride semiconductor power device including a doped nitride semiconductor zone for fixing avalanche breakdown.

FIG. 4 is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a nitride semiconductor power device including doped nitride semiconductor zones of same conductivity type for fixing avalanche breakdown.

FIG. 5 is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a nitride semiconductor power device including doped nitride semiconductor zones of different conductivity type for fixing avalanche breakdown.

FIG. 6 is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a nitride semiconductor power device including a doped nitride semiconductor zone and a trench contact for fixing avalanche breakdown.

FIG. 7 is a simplified flowchart illustrating one embodiment of a method for manufacturing a nitride semiconductor power device according to an embodiment.

FIGS. 8A to 8E are cross-sectional views illustrating one embodiment of a semiconductor portion during manufacture of a nitride semiconductor power device similar to the one illustrated in FIG. 1A.

FIGS. 9A to 9C are cross-sectional views illustrating one embodiment of a semiconductor portion during manufacture of the nitride semiconductor power device illustrated in FIG. 2.

FIG. 10A to 10D are cross-sectional views illustrating one embodiment of a semiconductor portion during manufacture of a nitride semiconductor power device similar to the ones illustrated in FIGS. 3 to 6.

DETAILED DESCRIPTION OF THE INVENTION

In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

FIG. 1A illustrates one embodiment of a schematic cross-sectional view of a semiconductor device, in one embodiment a portion of a lateral channel HEMT 100. HEMT 100 includes a carrier substrate 105 made of metal, e.g. Cu. On carrier substrate 105, an optional buffer layer 110 such as an MN buffer layer is arranged. On buffer layer 110, a first semiconductor layer 115 of Al_(x1)Ga_(y1)In_(z1)N (x1+y1+z1=1, x1≧0, y z1≧0), e.g. intrinsic GaN or GaN including at least one of Fe, C, Mg, in a concentration smaller than 5×10¹⁷ cm⁻³, or smaller 10¹⁸ cm⁻³ or smaller than 5×10¹⁸ cm⁻³ is arranged. On the first semiconductor layer 115, a second semiconductor layer 120 of Al_(x2)Ga_(y2)In_(z2)N (x2+y2+z2=1, x2>x1, y2≧0, z2≧0), e.g. AlGaN or InGaN, is arranged, either doped or undoped, optionally capped with a thin layer of GaN.

Source regions 125 a, 125 b and drain region 130 are electrically coupled to the second semiconductor layer 120. Gate regions 135 a . . . 135 d are arranged on the second semiconductor layer 120. The gate regions 135 a . . . 135 d may include metal and/or conductive semiconductor material such as doped polysilicon or p-doped GaN. The gate regions 135 a . . . 135 d may also include additional dielectric layers below the conductive gate region, e.g. similar to a MISFET (Metal-Insulator-FET). The conductivity between source and drain, e.g. between source region 125 a and drain region 130, may be controlled by applying a voltage to the gate regions 135 a . . . 135 d, e.g. gate region 135 b. An insulating layer 140, e.g. a Si₃N₄ or SiO₂ layer, is formed on the second semiconductor layer 120.

The drain region 130, which may include one or several conductive parts formed of metal such as Ti/Al or doped semiconductor material, is electrically coupled to the carrier substrate 105 and includes a conductive part extending through the second semiconductor layer 120, the first semiconductor layer 115 and the buffer layer 110 to the carrier substrate 105. The source regions 125 a, 125 b are electrically coupled to a contact area, e.g. a contact pad, at the front side. This contact pad may also be formed as part of a multi-metal layer system arranged on top of the active area of the device. According to another example, the source regions are electrically coupled to the carrier substrate at a rear side and the drain regions are electrically coupled to a contact area, e.g. a contact pad, at the front side.

A thickness of the carrier substrate 105 is appropriately chosen to provide mechanical stability to the layer stack arranged thereon. In addition, the carrier substrate supports dissipation of heat generated in the device arranged thereon in an operation mode of the device. As an example, the thickness of a carrier substrate 105 made of Cu may be between 15 μm to 50 μm, in particular between 30 μm to 40 μm. A metal layer of a same or different material than carrier substrate 105 may also be formed on the front side opposite to the rear side where the carrier substrate 105 is formed. In this case, each one of the carrier substrate at the rear side and the metal layer at the front side may contribute to the mechanical stability and may each have a thickness between 10 μm to 40 μm, in one embodiment between 20 to 30 μm. The metal carrier substrate(s) improve dissipation of heat during operation mode of the power semiconductor device(s) formed thereon. Omitting the buffer layer 110 may improve heat dissipation since this buffer layer which supports growth of GaN layers on initial silicon substrates may decrease heat dissipation due to a high thermal boundary resistance.

FIG. 1B illustrates a cross-sectional view of one embodiment of a semiconductor device including a portion of a lateral channel HEMT 100 that differs from the embodiment illustrated in FIG. 1A in that the source regions 125 a, 125 b are electrically coupled to a contact area, e.g. a contact pad 155 at the front side. The contact pad 155 is formed as part of a multi- or single-metal layer system including metals such as Cu arranged on top of the active area of the device. Furthermore, buffer layer 110 illustrated in FIG. 1A is omitted to improve heat dissipation as described above. An interlayer dielectric 160 may be provided to electrically insulate conductive regions from each other.

FIG. 2 is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a lateral channel HEMT 200. HEMT 200 differs from HEMT 100 illustrated in FIG. 1A in that buffer layer 110 is replaced by a third semiconductor layer 245 including Al_(x3)Ga_(y3)In_(z3)N (x3+y3+z3=1, x2>x3, y3≧0, z3≧0). The third semiconductor layer 245 is in contact with the first semiconductor layer 215 and includes an average concentration of dopants higher than 10¹⁷ cm⁻³.

In an OFF state, a vertical avalanche breakdown voltage between the second semiconductor layer 220, the first semiconductor layer 215 and the third semiconductor layer is set smaller than the lateral breakdown voltage between gate, e.g. gate region 235 d, and drain, e.g. drain region 230. Thus a channel region of HEMT 200 located at an interface 250 between the first semiconductor layer 215 and the second semiconductor layer 220 may be prevented from damage due to hot carrier degradation or other electrical stress mechanisms.

As an example, a distance 1 between gate, e.g. gate region 235 b, and drain, e.g. drain region 230, along a lateral direction 255 may be set larger than a thickness d of the first semiconductor layer 215 along a vertical direction 260 extending perpendicular to the lateral direction 255.

The concentration of dopants of the third semiconductor layer 245 may be chosen high enough to provide a beneficial ohmic contact to the carrier substrate 205, e.g. higher than 10¹⁷ cm⁻³, higher than 10¹⁸ cm⁻³ or even higher than 10¹⁹ cm⁻³. A conductivity type of the third semiconductor layer 245 may equal the conductivity type of the second semiconductor layer 220, e.g. both conductivity types being n-type or p-type. In another embodiment, the conductivity type of the third semiconductor layer 245 may differ from the conductivity type of the second semiconductor layer 220, e.g. the conductivity type of the third semiconductor layer 245 being p-type and the conductivity type of the second semiconductor layer being n-type.

FIG. 3 is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a lateral channel HEMT 300. HEMT 300 includes a carrier substrate 305 made of metal, e.g. Cu, doped Si such as n⁺-type Si, SiC or GaN, for example. On carrier substrate 305, a third semiconductor layer 345 is arranged. Third semiconductor layer 345 can be similar to third semiconductor layer 245 illustrated in FIG. 2. The carrier substrate 305 and the third semiconductor layer 345 constitute a drain of HEMT 300.

On third semiconductor layer 345, a first semiconductor layer 315 similar to first semiconductor layers 115, 215 illustrated in FIGS. 1A and 2 is arranged. On first semiconductor layer 315, a second semiconductor layer 320 of Al_(x2)Ga_(y2)In_(z2)N (x2+y2+z2=1, x2>x1, y2≧0, z2≧0), e.g. AlGaN, is arranged. In one or more embodiments, the second semiconductor layer 320 may be in the shape of stripes, columns, rings, hexagons, octagons and complementary structures, for example.

Gate regions 335 a, 335 b are arranged on the second semiconductor layer 320. The gate regions 335 a, 335 b may include metal and/or conductive semiconductor material such as doped polysilicon. The gate regions 335 a, 335 b may be congruent with the second semiconductor layer 320.

Source regions 325 a, 325 b are embedded in the first semiconductor layer 315, the source regions including a concentration of dopants higher than 10¹⁷ cm⁻³. The source regions 325 a, 325 b may be self-aligned to the second semiconductor layer 320 and the gate regions 325 a, 325 b.

A drift region 365 including a concentration of activated dopants higher than 10¹⁴ cm⁻³ extends through the first semiconductor layer 315. If the device is turned ON, the drift region provides a conductive path between the third semiconductor layer 345 and a channel region located at an interface 350 between the first semiconductor layer 315 and the second semiconductor layer 320. If the device is turned OFF, the drift region is partly depleted and contributes to the electrical isolation between source and drain. Shape and doping profile of the drift region may be chosen accordingly. The drift region 365 may include one doped semiconductor zone or a plurality of doped semiconductor zones overlapping each other in the vertical direction 360. In case of a plurality of doped and overlapping semiconductor zones, an average concentration within each of these zones may decrease in a direction from the third semiconductor layer 345 to the second semiconductor layer 320, for example, such as n⁻-type zone 366 and n-type zone 367 illustrated in FIG. 3.

The conductivity between source and drain, i.e. between source region 325 a and drift region 365, may be controlled by applying a voltage to the gate, i.e. gate region 335 a. An insulating layer 340, e.g. a SiN or SiO₂ layer, is formed on the second semiconductor layer 320 and the source regions 325 a, 325 b. Contact plugs 370 a, 370 b are formed within apertures of the insulating layer 340 and electrically couple the source regions 325 a, 325 b to a wiring level 375, e.g. a metal layer.

First avalanche regions 380 a, 380 b are formed within the first semiconductor layer 315, the first avalanche regions 380 a, 380 b being arranged opposite to the source, e.g. source regions 325 a, 325 b. The first avalanche regions 380 a, 380 b are in contact with the third semiconductor layer 345 and include an average concentration of activated dopants higher than 10¹⁷ cm⁻³.

In an OFF state, a vertical avalanche breakdown voltage between source, e.g. source region 325 a, and drain, e.g. third semiconductor layer 345, is set smaller than the lateral breakdown voltage between gate, e.g. gate region 335 b, and the drift zone 365, e.g. by appropriate choice of dimensions and dopant concentrations of first avalanche regions 380 a, 380 b. Thus a channel region of HEMT 300 located at the interface 250 between the first semiconductor layer 315 and the second semiconductor layer 320 may be prevented from damage by hot carrier degradation or other electrical stress mechanisms.

In one embodiment, a distance l₁ between source, e.g. source region 325 a, and an undepleted part of the drift region 365 along a lateral direction 355 may be set larger than a distance l₂ between a top side of the first avalanche regions, e.g. first avalanche region 380 a, and a bottom side of the source, e.g. source region 325 a, along a vertical direction 360 extending perpendicular to the lateral direction 355.

A threshold voltage Vth of HEMT 300 may be adjusted by choice of the gate material, the thickness of the second semiconductor layer 320, the concentration of dopants within the second semiconductor layer 320 and piezo-electric effects, for example. HEMT 300 may be a depletion mode transistor (Vth<0V) or an enhancement mode transistor (Vth>0V).

Source regions 325 a, 325 b, drift region 365 and first avalanche regions 380 a, 380 b may be formed by implanting dopants such as Si, Ge or O into the first semiconductor layer 315, for example. These regions may also be formed by epitaxial regrowth, for example. These regions may also have a same conductivity type, e.g. an n-type.

HEMT 300 exhibits an improved avalanche robustness. As a further example of arrangement of source and drain, both of them may be electrically coupled at a front side and the avalanche regions 380 a, 380 b may be electrically coupled to a contact region at a front side of the semiconductor device via the carrier substrate, a lead frame and a bond wire.

FIG. 4 is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a lateral channel HEMT 400. HEMT 400 differs from HEMT 300 illustrated in FIG. 3 in that second avalanche regions 482 a, 482 b are provided, the second avalanche regions 482 a, 482 b being arranged opposite to the first avalanche regions 480 a, 480 b. A conductivity type of the second avalanche regions 482 a, 482 b equals the conductivity type of the first avalanche regions. Dimensions and concentration of dopants of the second avalanche regions may be similar to the first avalanche regions 480 a, 480 b.

In an OFF state, a vertical avalanche breakdown voltage between source, e.g. source region 425 a, and drain, e.g. third semiconductor layer 445, is set smaller than the lateral breakdown voltage between gate, e.g. gate region 435 b, and the drift region 465, e.g. by appropriate choice of dimensions and dopant concentrations of the first and second avalanche regions 480 a, 480 b, 482 a, 482 b. In one embodiment, a distance l_(i) between source, e.g. source region 425 a, and an undepleted part of the drift region 465 along a lateral direction 455 may be set larger than a distance l₂ between the first and second avalanche regions 480 a, 480 b, 482 a, 482 b along a vertical direction 460 extending perpendicular to the lateral direction 455. Thus a channel region of HEMT 400 located at the interface 450 between the first semiconductor layer 415 and the second semiconductor layer 420 may be prevented from damage by hot carrier degradation or other electrical stress mechanisms.

FIG. 5 is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a lateral channel HEMT 500. HEMT 500 differs from HEMT 400 illustrated in FIG. 4 in that the second avalanche regions 582 a, 582 b are of opposite conductivity type than the first avalanche regions 580 a, 580 b. In one embodiment illustrated in FIG. 5, the second avalanche regions 582 a, 582 b are of p⁺-type and the first avalanche regions 580 a, 580 b are of n⁺-type. This arrangement is beneficial with regard to efficient discharge of avalanche current flowing into the second avalanche regions 582 a, 582 b, since this avalanche current is a hole current flowing into a p⁺-type region. In case of second avalanche regions 582 a, 582 b of n-type, the hole avalanche current will be injected into these regions and the injected holes will recombine within these regions. As an example, recombination centers may be formed within the second avalanche regions 582 a, 582, 482 a, 482 b and/or below a bottom side of these regions to enhance recombination of injected carriers in avalanche breakdown operation.

FIG. 6 is a cross-sectional view illustrating one embodiment of a semiconductor device including a portion of a lateral channel HEMT 600. HEMT 600 differs from HEMT 400 illustrated in FIG. 4 in that the first avalanche regions 482 a, 482 b are omitted and the contact plugs 670 a, 670 b partly extend into the source regions 625 a, 625 b and the second avalanche regions 682 a, 682 b as trench contacts.

In an OFF state, a vertical avalanche breakdown voltage between source, e.g. source region 625 a, and drain, e.g. third semiconductor layer 645, is set smaller than the lateral breakdown voltage between source, e.g. source region 625 a, and the undepleted part of the drift zone 665, e.g. by appropriate choice of dimensions and dopant concentrations of the second avalanche regions 682 a, 682 b and the trench contacts. As an example, a distance l₁ between source, e.g. source region 625 a, and drift region 665 along a lateral direction 655 may be set larger than a distance l₂ between the second avalanche regions 682 a, 682 b and the third semiconductor layer 645 along a vertical direction 660 extending perpendicular to the lateral direction 655.

FIG. 7 illustrates a simplified flowchart of a method for manufacturing a nitride semiconductor power device according to one embodiment.

At S100, a front side of a semiconductor body is attached to a first carrier, the semiconductor body including, in a sequence from a rear side to the front side, a semiconductor carrier substrate, a buffer layer including MN, a first semiconductor layer, in one embodiment made of Al_(x1)Ga_(y1)In_(z1)N (x1+y1+z1=1, x1≧0, y1≧0, z1≧0) and a second semiconductor layer in one embodiment made of Al_(x2)Ga_(y2)In_(z2)N (x2+y2+z2=1, x2>x1, y2≧0, z2≧0).

At S200, the semiconductor carrier substrate is removed from the rear side.

At S300, a metal substrate carrier is formed on the rear side.

The metal substrate carrier improves dissipation of heat generated during operation of a nitride semiconductor power device formed thereon.

FIGS. 8A to 8E are cross-sectional views illustrating one embodiment of a semiconductor device including a semiconductor portion, during one example of manufacture of a nitride semiconductor power device similar to the one illustrated in FIG. 1A.

Referring to the schematic cross-sectional view of FIG. 8A, a semiconductor body 800 is provided, the semiconductor body 800 including, in a sequence from a rear side to the front side, a semiconductor carrier substrate 803, e.g. a Si substrate, a buffer layer 810 including MN, a first semiconductor layer 815 of Al_(x1)Ga_(y1)In_(z1)N (x1+y1+z1=1, x1≧0, y1≧0, z1≧0) and a second semiconductor layer 820 of Al_(x2)Ga_(y2)In_(z2)N (x2+y2+z2=1, x2>x1, y2≧0, z2≧0). Source regions 825 a, 825 b and a drain region 830 are electrically coupled to the second semiconductor layer 820. Gate regions 835 a . . . 835 d are arranged on the second semiconductor layer 820. The gate regions 135 a . . . 135 d may include metal and/or An insulating layer 140, e.g. a SiN or SiO₂ layer, is formed on the second semiconductor layer 120.

Referring to the schematic cross-sectional view of FIG. 8B, the semiconductor body 800 is attached with its front side to a first carrier 880, e.g. a glass carrier or a metal carrier. Between the carrier 880 and the semiconductor body 800, an adhesive 882 may be provided. Then the semiconductor carrier substrate 803 is removed, e.g. by grinding or etching. Removal of the semiconductor carrier substrate 803 may be purely mechanical with a stop on the buffer layer 810 or may start with a mechanical removal process followed by an etching process.

Referring to the schematic cross-sectional view of FIG. 8C, a seed layer 885 such as a Cu seed layer may be formed on the buffer layer 810 at a rear side of the semiconductor body 800. The seed layer 885 may be formed by sputtering, for example, and also covers a bottom side of the drain region 830 extending through an insulating layer 840, the second semiconductor layer 820, the first semiconductor layer 815 and the buffer layer 810.

Referring to the schematic cross-sectional view of FIG. 8D, a metal is formed on the seed layer 885, e.g. by plating of Cu, to form a metal carrier substrate 810. The seed layer 885 may also be omitted and the metal, metal alloy or plurality of metal/metal alloys of the metal carrier substrate 810 may be formed on the buffer layer 810.

Referring to the schematic cross-sectional view of FIG. 8E, the semiconductor body 800 is attached, e.g. laminated, to a sawing foil 888 and the first carrier 880 as well as the adhesive 882 are released.

Further processes steps may follow to finalize nitride semiconductor power device including a metal substrate carrier for improved dissipation of heat during operation of the nitride semiconductor power device.

FIGS. 9A to 9C are cross-sectional views of one embodiment of a semiconductor portion during one example of manufacture of the nitride semiconductor power device similar to the one illustrated in FIG. 2.

Referring to the schematic cross-sectional view of FIG. 9A, a semiconductor body 900 is provided, the semiconductor body including a structure similar to the structure of semiconductor body 800 illustrated in FIG. 8A except that a third semiconductor layer including Al_(x3)Ga_(y3)In_(z3)N (x3+y3+z3=1, x2>x3, y3≧0, z3≧0) is arranged between the buffer layer 910 and the first semiconductor layer 915. Similar to the process described with reference to FIG. 8B, the semiconductor body 900 is attached with its front side to a first carrier 980, e.g. a glass carrier or a metal carrier. Between the carrier 980 and the semiconductor body 900, an adhesive 982 may be provided. Then the semiconductor carrier substrate 903 is removed, e.g. by grinding or etching. Removal of the semiconductor carrier substrate 903 may be purely mechanical with a stop on the buffer layer 910 or may start with a mechanical removal process followed by an etching process.

As is illustrated in FIG. 9B, after removal of the carrier substrate 903, the buffer layer 910 is removed, e.g. by plasma etching. Thus the third semiconductor layer 945 is exposed at a rear side of the semiconductor body 900.

Then, as illustrated in FIG. 9C, a seed layer 985 similar to the seed layer 885 described with reference to FIG. 8C is formed. The seed layer may also be a multi-layer metal stack for low ohmic contact purposes. Thereafter, the seed layer may be thickened by a metal or metal alloy and further processes may be carried out to end up with to a nitride semiconductor power device as illustrated in FIG. 2.

FIG. 10A to 10D are cross-sectional views illustrating one embodiment of a semiconductor device including a semiconductor portion during one example of manufacture of a nitride semiconductor power device similar to the ones illustrated in FIGS. 3 to 6. In particular FIGS. 10A to 10D illustrate one embodiment of formation of a lowermost zone of the doped drift region.

Referring to the schematic cross-sectional view illustrated in FIG. 10A, a doped third semiconductor layer of Al_(x3)Ga_(y3)In_(z3)N (x3+y3+z3=1, x3≧0, y3≧0, z3≧0) and a first semiconductor layer are formed on a semiconductor carrier substrate 155 made of Si, SiC or GaN, for example.

Referring to the schematic cross-sectional view illustrated in FIG. 10B, an aperture 191 is formed within the first semiconductor layer 165, e.g. by patterning a hard mask 198 on the first semiconductor layer 165 and etching through the first semiconductor layer 165 via the hard mask 198.

Then, as illustrated in the schematic cross-sectional view of FIG. 10C, a lowermost nitride semiconductor zone of the drift region 181 is formed within aperture 191 by selective epitaxy. This zone 181 may be doped in-situ to include a concentration of activated dopants higher than the first semiconductor layer 165.

Then, as illustrated in the schematic cross-sectional view of FIG. 10D, a top surface of the lowermost nitride semiconductor zone of the drift region 181 is made planar, e.g. by chemical mechanical polishing (CMP). Thereby the hard mask 198 may be removed. A barrier layer 199 may be formed on the surface of the leveled surface.

The sequence of processes described with regard to FIGS. 10A to 10D may be repeated to form a plurality of doped semiconductor zones, e.g. avalanche regions, source, drain regions. As a further example, by repeating above sequence of processes, a continuous semiconductor region may be formed by a plurality of semiconductor zones in contact with each other, wherein the semiconductor zones may differ with regard to their dopant concentration. Thereby, a desired vertical dopant profile may be achieved, for example. As an example, a drift region may be formed by a plurality of doped semiconductor zones having different concentrations of dopants such that the concentration of dopants decreases from a rear side, i.e. at an interface with the third semiconductor layer 195, to the front side, cf. drift region 365 including zone 366 and 367 illustrated in FIG. 3.

It is to be understood that the features of the various embodiments described herein may be combined with each other unless specifically noted otherwise.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptions or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof. 

1. A semiconductor device, comprising a carrier substrate including metal; a first semiconductor layer of Al_(x1)Ga_(y1)In_(z1)N (x1+y1+z1=1, x1≧0, y1≧0, z1≧0) above the carrier substrate; a second semiconductor layer of Al_(x2)Ga_(y2)In_(z2)N (x2+y2+z2=1, x2>x1, y2≧0, z2≧0) on the first semiconductor layer; a gate region on the second semiconductor layer; and a source region and a drain region, wherein one of these regions is electrically coupled to the carrier substrate and includes a conductive region extending through the first semiconductor layer.
 2. The semiconductor device of claim 1, further comprising a third semiconductor layer including MN between the carrier substrate and the first semiconductor layer.
 3. The semiconductor device of claim 1, further comprising a third semiconductor layer including Al_(x3)Ga_(y3)In_(z3)N (x3+y3+z3=1, x2>x3, y3≧0, z3≧0) between the carrier substrate and the first semiconductor layer and in contact with the first semiconductor layer, the third semiconductor layer including an average concentration of dopants higher than 10¹⁷ cm⁻³.
 4. The semiconductor device of claim 1, wherein the carrier substrate is made of Cu.
 5. The semiconductor device of claim 1, wherein the conductive region includes a metal.
 6. The semiconductor device of claim 1, wherein a distance between the gate region and the drain region along a lateral direction extending parallel to an interface between the first and second semiconductor layers is larger than a thickness of the first semiconductor layer along a vertical direction extending perpendicular to the interface.
 7. The semiconductor device of claim 1, wherein both the source region and the drain region include doped semiconductor regions of a same conductivity type within the first semiconductor layer, an average concentration of dopants within each of these regions being higher than 10¹⁷ cm⁻³.
 8. The semiconductor device of claim 7, wherein the second semiconductor layer is formed on a first part of the first semiconductor layer including the conductive region and is absent on a second part of the first semiconductor layer including the source region.
 9. The semiconductor device of claim 7, further comprising a doped semiconductor region within the first semiconductor layer, wherein an average concentration of dopants of the doped semiconductor region is higher than 10¹⁷ cm⁻³, the doped semiconductor region being formed at a rear side of the first semiconductor layer and opposite to the source region at the front side of the first semiconductor layer.
 10. The semiconductor device of claim 7, further comprising a doped semiconductor region within the first semiconductor layer, wherein an average concentration of dopants of the doped semiconductor region is higher than 10¹⁷ cm⁻³, the doped semiconductor region overlapping a bottom side of the source region.
 11. The semiconductor device of claim 1, wherein the conductive region includes a doped epitaxial layer formed within an aperture of the first semiconductor layer.
 12. A semiconductor device, comprising a carrier substrate; a first semiconductor layer of Al_(x1)Ga_(y1)In_(z1)N (x1+y1+z1=1, x1≧0, y1≧0, z1≧0) above the carrier substrate; a second semiconductor layer of Al_(x2)Ga_(y2)In_(z2)N (x2+y2+z2=1, x2>x1, y2≧0, z2≧0) on the first semiconductor layer; a gate region on the second semiconductor layer; a source region and a drain region; and a third semiconductor layer including Al_(x3)Ga_(y3)In_(z3)N (x3+y3+z3=1, x2>x3, y3≧0, z3≧0) between the carrier substrate and the first semiconductor layer and in contact with the first semiconductor layer, the third semiconductor layer including an average concentration of dopants higher than 10¹⁷ cm⁻³.
 13. The semiconductor device of claim 12, wherein one of the source region and the drain region is electrically coupled to the carrier substrate and includes a conductive region extending through the first semiconductor layer.
 14. The semiconductor device of claim 12, wherein the carrier substrate at a rear side of the semiconductor device is electrically coupled to a contact region at a front side of the semiconductor device via a lead frame and a bond wire.
 15. The semiconductor device of claim 12, wherein the carrier substrate includes at least one of doped Si, SiC, GaN, metal.
 16. The semiconductor device of claim 12, further comprising a doped semiconductor region within the first semiconductor layer, wherein an average concentration of dopants of the doped semiconductor region is higher than 10¹⁷ cm⁻³, the doped semiconductor region being formed at a rear side of the first semiconductor layer opposite to the source region at a front side of the first semiconductor layer.
 17. The semiconductor device of claim 12, further comprising a doped semiconductor region within the first semiconductor layer, wherein an average concentration of dopants of the doped semiconductor region is higher than 10¹⁷ cm⁻³, the doped semiconductor region overlapping a bottom side of the source region.
 18. The semiconductor device of claim 12, wherein a distance between the gate region and the drain region along a lateral direction extending parallel to an interface between the first and second semiconductor layers is larger than a thickness of the first semiconductor layer along the vertical direction extending perpendicular to the interface.
 19. The semiconductor device of claim 12, wherein the conductive region includes a doped epitaxial layer formed within an aperture of the first semiconductor layer.
 20. The semiconductor device of claim 12, wherein the first semiconductor layer includes at least one of Fe, C, Mg.
 21. A method for manufacturing a semiconductor device, comprising: attaching a front side of a semiconductor body to a first carrier, the semiconductor body including, in a sequence from a rear side to the front side, a semiconductor carrier substrate, a buffer layer including MN, a first semiconductor layer of Al_(x1)Ga_(y1)In_(z1)N (x1+y1+z1=1, x1≧0, y1≧0, z1≧0) and a second semiconductor layer of Al_(x2)Ga_(y2)In_(z2)N (x2+y2+z2=1, x2>x1, y2≧0, z2≧0); removing the semiconductor carrier substrate from the rear side; forming a metal substrate carrier on the rear side.
 22. The method of claim 21, wherein forming the metal substrate carrier includes forming a seed layer of Cu on the rear side; and forming Cu on the rear side by galvanic plating.
 23. The method of claim 21, wherein the semiconductor body comprises a third semiconductor layer including Al_(x3)Ga_(y3)In_(z3)N (x3+y3+z3=1, x2>x3, y3≧0, z3≧0) between the carrier substrate and the first semiconductor layer and in contact with the first semiconductor layer, the third semiconductor layer including an average concentration of dopants higher than 10¹⁷ cm⁻³; the method further comprising removing the buffer layer after removal of the semiconductor carrier and before formation of the metal substrate carrier.
 24. The method of claim 21, further comprising forming an aperture at least within the first semiconductor layer; and forming a conductive material within the aperture.
 25. The method of claim 22, wherein forming the conductive material within the aperture includes forming a doped epitaxial semiconductor layer within the aperture. 